Interference reducing radio pulse receiver



April 12, 1949.

C, W. HAN SELL INTERFERENCE REDUCING RADIO PULSE RECEIVER Filed March17, 1945 Patented Apr. 12, 1949 INTERFERENCE REDUCING RADIO PULSERECEIVER Clarence W. Hansell, Port Jefferson, N. Y., assgnor to RadioCorporation of America, a corporation of Delaware Application March 17,1945, Serial No. 583,239

(Cl. Z50-20) Claims.

1 This invention relates to pulse radio signalling systems, and moreparticularly to an improved .pulse radio receiver for improving thereception of pulses of radio frequency energy.

An object of the present invention is to provide a pulse radio receiverwhich is characterized by an improved signal-to-noise ratio.

Briey stated, the pulse receiver of the invention utilizes a degree ofselectivity before the final detector so great as to lose or distort theoriginal Wave form of the signal pulse and then by suitable apparatuscorrect the modulation frequency characteristic after detection so as torestore the original wave form of the signal pulse so far as may benecessary. This is accomplished by employing a superheterodyne receiverin which the inter-- mediate frequency circuits are designed to be tooselective to pass pulses undistorted, and by passing the rectied pulsesthrough an equalizer or compensator having a characteristic which isopposite to that of the highly selective intermediate frequency circuitswithin the band of frequencies which it is desired that the receiver asa Whole be capable of utilizing. The receiver is thus able to operate atlower signal levels before the signal pulses are destroyed by noise; or,putting it in other words, the signal threshold is lowered to a valuebelow which the noise blots out or masks the signal. It should beunderstood, however, that the receiver of the invention operates withsubstantially the same overall band width and sensitivity to tuning inthe case of good signal-to-noise ratio as presently used on conventionaltype pulse receivers.

An explanation of the theory underlying the present invention will nowbe given in order that the invention may be better appreciated. Whilethis theoretical explanation is believed to be correct, it is not ofnecessity complete, nor does the operation of the invention depend uponits accuracy or otherwise.

The depression of the "improvement threshold by increasing theintermediate frequency selectivity is possible because thesignal-to-noise ratio per frequency interval is not constant throughoutthe frequency band occupied by the pulse, but is a maximum at the centerof the pulse frequency spectrum and tapers to a. minimum at the limitsof the spectrum. Because of this unequal distribution of signal-to-noiseratio per frequency interval, the total signal-to-noise ratio of currentreaching the detector throughout the whole frequency band lssubstantially less than the signalto-noise ratio for frequencies at themiddle of the band. By increasing the amplitude of cur..

. 2 rents at the middle of the band, Where the signalto-noise ratio isbest, with respect to currents in other parts of the band where thesignal-to-noise ratio is Worse, I am able to provide at the detector animproved overall signal-to-noise ratio. This improvement in overallsignal-to-noise ratio automatically depresses the improvement thresholdand enables the receiver to reach down to weaker signals before thesignals are destroyed due to masking the signal in the output of thedetector by noise in the input to the detector.

By masking is meant the observed destruction of useful signal currentsin the output of a rectier due to the presence of strong noise in theinput to the detector. It is an experimental fact that thesignal-to-noise ratio in the output of the detector may be less than thesignal-to-noise ratio in the input to the detector when the input signalis below a so-called improvement threshold" level. This improvementthreshold level is determined by the total amount of noise currents inthe input to the detector which in turn is dependent on the frequencyselectivity of circuits preceding the detector.

The theoretical ultimate in retrieving signals from the noise will bereached when the selectivity is provided by resonant circuits of zeroresistance tuned to the frequency of the pulse carrier current. Inemploying a theoretically zero loss circuit before the detector, acircuit responsive only to rate of change of detector output currentmust be used to correct the overall pulse response to avoid loss of Waveform. With this theoretically zero loss circuit, the signal-to-noiseratio at the input of the detector will have reached an ultimate limit.This ultimate limit is equal to the signalto-noise ratio per frequencyinterval at the exact center of the pulse spectrum for a band of zerolimiting width. I thus furnish a new definition of the theoreticalultimate lower signal limit of a receiver as one in which the maximumsignalto-noise ratio at any place in the band determines the point atwhich signals are masked by the noise. For radio locating purposes, Iconsider the limit of low signal strength to be obtained when at thatbest point in the frequency band, signal and noise have equal R. M, S.values at the input to the detector.

From one point of view, my use in a radio pulse receiver of excessiveselectivity ahead of the detector with correction of the frequencycharacteristic after detection, is a form of, or analogous to, carrierexaltation. The use of excessive frequency selectivity in a broadcastreceiver serves to reduce the percentage modulation of the carriercurrent by the combination of signal side frequencies and noise, and tomaintain it below 100% down to lower initial signal-to-noise ratlos.These same principles apply to the reception of any modulated carriercurrent including pulse modulated carrier currents. Stated in anotherway, in any communications system where the intelligence is carried bythe variations or modulations in carrier current, the possibility of theintermediate frequency currents (including both signals and noise)reaching the detector rectifier exceeding 100% amplitude modulation ofthe carrier current should be prevented if the intelligence modulationsare to be preserved.

The pulse radio receiver of the invention is useful in radio locators,sometimes called radar or object detection systems, which function bydetecting the radio echo or pulse reflected by any object which theradar beam strikes. By narrowing the pass band of the intermediatefrequency circuits of the radar receiver, so as to reduce the area underthe normal frequency selectivity curve to half, and then by correctingthe overall frequency response by circuits following the detector, it isbelieved that there often will be achieved a lowering of the minimum ofuseful signal level by about 2:1 in power, which should result in 21Agreater range of detection and enable protecting an area 21/2 timeslarger than conventional radar receivers. Heretofore, the conventionaldesign of a radar receiver would not permit depressing the improvementthreshold and thereby gaining an increase in the ultimate range to anysubstantial degree by lengthening the pulses to increase the transmittedpower. The reason for this is that pulses of the present length alreadyrise to maximum amplitude in the input circuits of the rectifyingdetectors, and making them longer will not raise the peak amplitude anyhigher with respect to noise. However, by making the intermediatefrequency circuits too selective to reproduce the wave form and lengthof the pulses in accordance with the present invention, then, over arange,

' lengthening the transmitted pulses can be made to increase theamplitude of signal with respect to noise in proportion to the length ofthe signal i pulse and the improvement threshold will be depressed by2:1 in amplitude, or 4:1 in power when the pulse length is doubled. Fromthe ordinary point of view, this comes about because the averagetransmitted power has been doubled and at the same time its frequencyband width reduced to one-half.

The invention is also useful in pulse communications systems regardlessof the type of modulation employed; that is, whether the radio frequencyenergy constituting the pulses is itself modulated, or the timing orphase of the pulses is modulated. The most general application of thepulse radio receiver of the invention for communication purposes will bein connection with systems wherein the pulse timing or phase ismodulated at the remote transmitter.` This pulse receiver isparticularly adapted for use in any of the foregoing systems wherein thetransmitted pulsesl are short compared to the time intervals betweenthem, and greater peak power employed at the transmitter compared to theaverage power.

The following is a more detailed description of the invention inconjunction with drawings, wherein Fig. 1 illustrates one embodiment ofthe invention; Figs. 2a and 2b illustrate the detalls of different typesof pulse form restorer circuits employed in Fig. 1; and Figs. 3a, 3b and3c are amplitude versus frequency curves given to graphically illustratethe operation of the present invention.

Referring to Fig. 1 in more detail, there is shown, in box form, a pulseradio receiver adapted to receive pulses which are short compared to thetime intervals between them. This receiver is of the superheterodynetype and includes a receiving antenna I, a radio frequency amplifier 2,a frequency converter 3 which is supplied both with the output of theamplifier 2 and with energy from a local heterodyning oscillator 4, andintermediate frequency amplier circuit 5 of one or more stages.Amplifier 5 is designed to be too selective to retain the pulse waveform supplied to its input circuit; or, putting it in other Words, itprovides excessive selectivity ahead of the pulse rectifier 6 which issupplied with the output from the amplifier 5.

This excessive selectivity in the intermediate frequency amplifier 5 maybe provided by suitable frequency selective transformers, or, wheredesired, by a crystal filter circuit. For greatest benefit, the highestpossible selectivity should be furnished by the intermediate frequencyamplilier 5, provided this selectivity can be represented by a more orless smooth curve throughout the frequency band occupied by thetransmitted pulses. The output of the pulse rectier 6 is in the form ofunidirectional current pulses and is supplied to a pulse form restorer 1which in effect is an equalizer or compensator having a characteristicwhich within the signal band is substantially opposite to that of theexcessively selective intermediate frequency amplifier circuit 5. Outputfrom the pulse form restorer 'l is supplied to leads A and B, and thisoutput may, in turn, be passed to a pulse demodulator 8 followed by anaudio amplifier 9 when the system is used for general communicationpurposes, or, in the alternative, the output from leads A and B may bepassed on through an amplifier to a cathode ray oscilloscope system I0when the system is used for radio locating or object detection purposes.

The most advantageous application of the system of Fig. 1 when used forgeneral communication purposes, is in a system in which the timing orphase of the pulses is modulated by the signal intelligence at theremote transmitter, in which case the demodulator 8 would be a.demodulator of phase modulated pulses.

Where the system of Fig. 1 is used for object detection or radiolocating purposes, thecathode ray oscilloscope I0 will be provided withsuitable sweep circuits and electron defiection electrodes forindicating on the screen of the tube the distance from the receivingapparatus of the object which is being detected, and in some cases bothdirection and distance, in accordance with the principles of radarsystems now in use.

In radio locating systems of which the -system of Fig. 1 may constitutean essential part, the receiver and its transmitter are usually locatedtogether. It is customary to transmit extremely short pulses of ultrahigh frequency energy of the order of ve microseconds down to. a smallfraction of a mlcrosecond, depending upon the type of systemk involved,and to employ pulse repetition rates ranging from to several thousandpulses per second. The receiver is customarily blocked by apparatus (notshown) so as to be non-responsive during the intervals in which pulsesare actually transmitted, and the receiver isfdesigned to be responsiveor receptive during .the time intervals between the transmitted pulses.

The same antenna is preferably used both for transmitting and forreceiving purposes, and this antenna, which is shown diagrammaticallyonly in Fig. 1, may be of the lobe switching or scanning .type in whichpulses are transmitted synchronously with the lobe switching, while theantenna may be made to rotate during the hunting for objects. Obviously,for radio locating purposes (radar), the antenna is directive in nature,and

-where the system is employed to show both direction and distance of anynumber of distant objects by means of the oscilloscope presentation, theantenna may be rotated at a constant rate during each rotation of whichmany pulses may be transmitted. One suitable form of radar System inwhich the present invention may be employed is illustrated and describedin copending application Serial No, 454,661, led by Nils E. Lindenbladon August 13, 1942.

Where the system of Fig. 1 is used for general communication purposes,such as for ordinary narrow band telephone, a pulse rate of 10,000'persecond may be sucient for a single channel system, whereas forbroadcasting purposes a pulse rate of 30,000 to 50,000 per second shouldbe aqequate.

Figs. 2a and 2b illustrate, by way of example only, two diiferent typesof pulse form restorer circuits which can be used in box I of Fig. 1.Vari- 'ous combinations of these two circuits and other known types ofequalizers or compensators can also be employed in place of either oneof the circuits of Figs. 2a or 2b. In fact, there is a vast developedart in the field of equalizers for obtaining almost any desired overallresponse charac'- teristics in electrical circuits.

Fig. 3a illustrates, by way of example only, the operation of the highlyselective circuit of the intermediate frequency amplifier 5. In thisiigure, the rectangular curve 20 illustrates generally the band widthand the amplitude versus frequency response characteristic necessary topass, without distortion of the Wave form, pulses of the lengthtransmitted from the remote transmitter. The curve 2| illustrates theform of frequency versus amplitude curve of the highly selective circuit5, as a result of which the central portion of the frequency band hasthe amplitude of its currents greatly increased with respect to theouter portions.

Fig. 3b illustrates one form of typical amplitude Versus frequency curvewhich may be characteristic of the pulse form restorer l in order torestore the pulse passed by the high frequency amplier 5 to its originalform. It should be noted that. the characteristic of the pulse formrestorer 1 is such that the amplitude increases with increase infrequency, so as to be opposite in its effect upon modulation to that ofthe highly selective intermediate frequency circuit on one side of thecentral or mean frequency. The phase response, or time delay, are alsomade to compensate.

Fig. 3c illustrates the overall frequency response of the receiver up tothe output of the pulse form restorer 1. It should be noted that theresponse shown in Fig. 3c is similar in shape to one-half the size ofthe pulse 20 of Fig. 3a because (after rectication) the frequencyresponse begins at zero frequency corresponding to the carrierfrequency.

r The frequency response curves have their corresponding efects upon thewave form of the signal pulses. The circuits with the frequency response2| of Fig. v3a tend to stretch out the pulses in time so that they arebroader than the transmitted pulses. After detection, the circuits withthe response shown in Fig. 3b shorten the pulses again in order torestore them to shapes substantially like those of the transmitted waveforms, as though the selectivity had been that of 20 of Fig. 3a.

10 However, the altered relation of signal-tonoise, in the input to thepulse rectifier of Fig. l, improves the average signal-to-noise ratioand allows successful detection of the pulses down to a lower powerlevel.

15 The arrangement of the invention is particularly effective againstcontinuous wave interference from an interfering current within one edgeof the intermediate frequency pass band.

l What is claimed is:

20. 1l. A radio receiver of the superheterodyne type` if for. receivingpulses of radio carrier current which are repeated and which are ofshort time duration compared to the time intervals between them,comprising an intermediate frequenc amplifier, a rectifier forconvertingarl'urrent pu ses in the output of the intermediate frequencyamplier into direct current pulses, and a pulse wave form restorerfollowing said rectier,said intermediate frequency amplier being sodesigned and adjusted as to greatly emphasize or increase the amplitudesof components of current having frequencies near the center of thefrequency band occupied by the carrier current pulses with respect tothe amplitude of components of current having frequencies on either sideof the center of said band, and said pulse restorer having an amplitudeversus frequency response characteristic which increases higherfrequency components of the rectified direct current pulses with respectto lower frequency components, within the modulation frequency bandoccupied by the pulses, thereby restoring to the rectified directcurrent pulses the same approximate wave form as that of the envelope ofthe carrier current pulses.

2. A pulse radio receiver adapted for use with a remote transmitterradiating pulses of radio frequency energy which are modulated in timingby the intelligence to be carried and which are of short durationcompared to the time intervals between them, comprising an energycollector, a

radio frequency amplier coupled to said coll lector, a frequencyconverter coupled to the output of said radio frequency amplifier, alocal generator of oscillations coupled to said converter, a highlyselective intermediate frequency amplifier coupled to the output of saidconverter, a pulse rectier for the output of said intermediate frequencyamplifier, and a pulse form restorer circuit following said rectier,said intermediate frequency amplier being so designed and adjusted as togreatly emphasize or increase the amplitudes of components of currenthaving frequencies near the center of the frequency band occupied by thecarrier current pulses with respect to the amplitude of components ofcurrent having frequencies on either side of the center of said band,and said pulse restorer having an amplitude versus frequency responsecharacteristic which increases higher frequency components of therectified direct current pulses with respect to lower frequencycomponents, within the modu.- lation frequency band occupied by thepulses. a demodulator of phase modulated pulses coupled to the output ofsaid pulse form restorer, and an 7 audio frequency translator circuitcoupled to the output of said demodulator.

3. In a radio object detection system which radiates pulses of ultrashort wave energy which are short in duration compared to the timeintervals between them, a superheterodyne receiver including anintermediate frequency so designed and adjusted as to greatly emphasizeor increase the amplitudes of components of current having frequenciesnear the center of the frequency band occupied by the carrier currentpulses with respect to the amplitude of components having frequencies oneither side of the center of said band, a rectifier coupled to theoutput of said intermediate frequency amplifier, and a pulse wave formrestorer coupled to the output of said rectiiier for substantiallyrestoring to the rectified direct current pulses the same approximatewave form as that of the envelope of the carrier current pulses, saidrestorer having an amplitude versus frequency response characteristicwhich increases higher frequency components of the rectified directcurrent pulses With respect to lower frequency components, and a cathoderay tube indicator circuit coupled to said pulse Wave form restorer.

4. In a system wherein there are transmitted signal pulses of radiofrequency energy which are of short duration compared to the timeintervals between them, a superheterodyne receiver including anintermediate frequency ampliiier so designed and adjusted as to greatlyemphasize or increase the amplitudes of components of current havingfrequencies near the center of the frequency lband occupied by thecarrier current pulses with respect to the amplitude of componentshaving frequencies on either side of the center of said band, saidintermediate frequency amplier having suflicient response to pass twicethe band o ccupied by the modulation frequencies,

. rent signal pulses having an envelope of approximately rectangularshape by noise in pulse communications receivers, comprising circuitscoupled to the input and output of a detector for converting carriercurrent pulses to direct current pulses, the circuits coupled to theinput of the detector being so selective as to emphasize the amplitudeof components of currents in the center of the frequency band occupiedby the carrier current pulses with respect to those at the sides of theband, and the circuits coupled to the output of the detector having anamplitude versus frequency response characteristic which increases theamplitude of higher frequency components with respect to lower frequencycomponents, thereby toA correct for distortions in wave form and lengthofV the-detected pulses caused by the selectivity of the circuitscoupled to the input of the detector.

CLARENCE W. HANSELL.

REFERENCES CITED The following references are of record in the iile-"ofthis patent:

UNITED STATES PATENTS Number Name Date 1,922,139 Nyquist Aug. 15, 19331,936,153 Burton Nov. 21, 1933 1,961,334 Burton June 5, 1934 2,113,214Luck Apr. 5, 1938 2,122,990 Poeh July 5, 1938 2,184,978 Nyquist Dec. 26,1939 2,281,996 Randall et al. May 5, 1942 2,281,997 Randall May 5, 19422,337,196 Hollingsworth Dec. 21, 1943 2,416,328 Labin Feb. 25, 1947OTHER REFERENCES Selective Circuits and. Static Interference, byk

J. R. Carson, Transactions of A. I. E. E.. June 1924, pp. 789-796.

Multichannel Communication Systems, by F. F. Roberts and J. C. Simmonds,Wireless Engineer, Nov. 1945, pp. 538-549.

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Pulse-Time Modulation, by E. M. Deloraine and E. Labin, Electronics,Jan. 1945, pp 10D-105..

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